Epidemic viral infections are responsible for significant worldwide loss of life and income in human illnesses ranging from the common cold to life-threatening influenza, West Nile, and HIV infections. Timely detection, diagnosis and treatment are key in limiting spread of disease in epidemic, pandemic and epizootic settings. Rapid screening and diagnostic methods are particularly useful in reducing patient suffering and population risk. Similarly, therapeutic agents that rapidly inhibit viral assembly and propagation are particularly useful in treatment regimens.
Influenza has emerged as a potentially significant risk to human populations. Influenza can infect as much as 5-15% of the world population annually, resulting in 3-5 million cases of severe illness and up to 500,000 deaths each year. In the U.S. alone, flu epidemics lead to approximately 300,000 influenza-related hospital admissions and 36,000 influenza-related deaths annually in addition to an estimated cost of $12 billion per year. Avian strains have also crossed into humans, and there is growing evidence that the human-to-human transmission of avian strains has occurred in the past. The pandemic strains can kill far more people, for example, the 1918 strain has been reported to kill ˜50 million people worldwide, more than people killed during the first world war.
Virology test methods for detection and confirmation of influenza infection in a virus-secure reference laboratory (e.g., satisfying requirements for Containment Group 2/3/4 pathogens) are time consuming, high-risk and laborious (i.e., involving 4-7 days isolation of the virus in embryonated eggs; harvesting allantoic fluids from dead or dying embryos; testing the fluid in hemagglutination and hemagglutination inhibition tests, immunodiffusion; and, eventual subtyping of the virus in the fluid by hemagglutinin and neuraminidase in overnight immunodiffusion assays using specially prepared monospecific antisera). Present subtyping of influenza involves identifying each of 16 different possible viral hemagglutinin proteins in combination with 9 different possible viral neuraminidase proteins.
Rapid immunodiagnostic tests for influenza antigens have been developed, and include BINAXNOW FluA and FluB (Binax, Inc., Portland, Me.), DIRECTIGEN Flu A+B (Becton Dickinson, Franklin Lakes, N.J.), FLU OIA (Biostar Inc., Boulder, Colo.), QUICKVUE (Quidel, Sand Diego, Calif.), INFLU AB QUICK (Denka Sieken Co., Ltd., Japan) and XPECT FLU A & B (Remel Inc., Lenexa, Kans.). These assays can reportedly either detect influenza A or distinguish between Influenza A and B, but importantly, not between different influenza A subtypes or between pathogenic and non-pathogenic strains of influenza A. Moreover, these tests cannot determine resistance to FDA approved antivirals and exhibit high false negative results with emerging strains.
Recent introduction of reverse-transcriptase PCR-based diagnostics (RT-PCR) for confirming influenza A virus have resulted in important advances in diagnostics, but because of the relative inefficiency of the reverse transcriptase enzyme and significant amounts of virus required (e.g., 104 virion particles), high throughput screening of subjects with RT-PCR in an epidemic setting is not practical. These tests are typically performed in clinical laboratory by trained personnel.
Additionally, the complexity, diversity and rapid emergence of new influenza strains has made the diagnosis of high risk strains difficult using conventional approaches. For epidemiologists, diversity resulting from high mutation rates and genetic reassortment make it challenging to anticipate where new strains may originate. Thus, there remains a significant need in the medical arts for improved, inexpensive, rapid, accurate and discriminatory methods capable of detecting influenza and identifying influenza strains, particularly in point-of-care settings, as well as determining the antiviral resistance of influenza strains.